Specific Speed
1. Specific speed definition
The specific speed of water pump is abbreviated as specific speed, which is usually represented by the symbol ns. The specific speed and the rotational speed are two completely different concepts. The specific speed is a comprehensive data calculated by using the basic parameters Q, H, N, which indicate the characteristics of water pump. It can also be called comprehensive criterion. It is closely related to the structural shape of pump impeller and the performance of pump.
Calculation formula of Specific Speed in China
Calculation formula of Specific Speed abroad
1. Q and H refer to the flow rate and head at the highest efficiency, and n refers to the design speed. For the same pump, the specific speed is a certain value .
2. Q and H in the formula refer to the design flow rate and design head of single-suction single-stage pump. Q/2 is substituted for double suction pump; For multi-stage pumps, the head of the first-stage impeller should be substituted for calculation .
| Pump style | Centrifugal pump | Mixed-flow pump | Axial flow pump | ||
|---|---|---|---|---|---|
| Low specific speed | Medium specific speed | High specific speed | |||
| Specific speed |
30 |
80 |
150 |
300 |
500 |
1. A pump with low specific speed means high head and small flow, while a pump with high specific speed means low head and large flow.
2. The impeller with low specific speed is narrow and long, and the impeller with high specific speed is wide and short.
3. The low specific speed pump is prone to hump.
4, low specific speed pump, the shaft power is small when the flow is zero, so close the valve to start. High specific speed pumps (mixed flow pump, axial flow pump) have large shaft power at zero flow, so open the valve to start.
Specific revolutions and allowable cutting amount
|
ns |
60 | 120 | 200 | 300 | 500 |
|---|---|---|---|---|---|
|
|
0.2 | 0.15 | 0.11 | 0.09 | 0.07 |
